The invention relates to a system for charging batteries, particularly nickel cadmium batteries. The invention relates more particularly to very small, compact battery charger circuits suitable for charging batteries of communications products without introducing electrical noise that may interfere with operation thereof.
A fully charged state of a nickel cadmium battery is achieved by controlling the charging when the terminal voltage falls or "droops" by a certain amount (for example, 100 millivolts) from its peak value during high current charging. Nickel cadmium batteries are known which are able to withstand a relatively high charging rate. A high battery charging rate is desirable in order to reduce the charging time and therefore the amount of time a battery is out of service. For a typical fast charge, a current that is numerically equal in amperes to the battery capacity in ampere hours is supplied to the battery for approximately one hour. It is known that high current charging of a nickel cadmium battery should be stopped soon after the onset of a negative rate of change of the battery voltage. There are known battery chargers that automatically sense a fully charged condition of a battery and then terminate the main charging current produced by the charger and substitute a trickle current. As the battery reaches full charge, the charging rate is reduced to a trickle charge or stopped. It is important to control the cutoff of charging so as to assure that the battery has been fully charged, and also to prevent overcharging that may damage the battery cells.
It is known that many communications products, such as portable cellular telephones, are highly sensitive to presence of electrical noise. Prior battery chargers generally introduce a substantial amount of electrical noise onto conductors connected to the terminals of the battery being charged. Furthermore, radiated high frequency interference may be picked up by rf amplifiers. If a communications product such as a portable cellular telephone is being used while its battery is being charged, such electrical noise is likely to deleteriously affect performance of the cellular telephone.
It would be highly desirable to provide a compact, low noise battery charger with low power dissipation that could be incorporated easily in a communications product or a power cord thereof to continually charge nickel cadmium batteries whenever the power cord is connected to a source of AC line current. Prior battery chargers which are inexpensive enough for this purpose unfortunately require a long (e.g., twelve hours) charging time. More elaborate "fast" battery chargers are expensive, large, consume too much power, and/or generate too much electrical noise to be used simultaneously in most communication products.
There is a presently unmet need for a compact, very low noise, inexpensive battery charger suitable for charging nickel cadmium batteries of noise-sensitive products, while allowing such products to be used while battery charging is occurring, without noise-caused operating problems.